Issue #11904 has been updated by Charles Nutter.


Nobuyoshi Nakada wrote:
> This use of `Thread.exclusive` seems nonsense.
> https://github.com/cryptosphere/rbnacl/commit/25444979d161fa0f02a61a5003768c2173cfa2ee
> 
> While `Thread.exclusive` stops all other threads in 1.8 and earlier, it just blocks other threads which also calling it.
> If no other threads calls it, it is useless.

That's the point; no two threads can call init via this code at the same time.

Of course, require's guarantee of "only once" would do the same thing here.

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Misc #11904: Why was Thread.exclusive deprecated?
https://bugs.ruby-lang.org/issues/11904#change-55972

* Author: Tony Arcieri
* Status: Rejected
* Priority: Normal
* Assignee: 
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See: https://bugs.ruby-lang.org/projects/ruby-trunk/repository/revisions/52554

Why was Thread.exclusive deprecated? It is useful for when you're uncertain about whether the caller is multithreaded or not, and therefore cannot initialize a mutex because the mutex must be initialized in a thread-safe context where it's not possible for multiple caller threads to initialize the mutex concurrently.

One use case is here: this is an idempotent native function invoked via FFI. The contract is that it can be called repeatedly, but only by one thread at a time (concurrent calls from multiple threads can potentially corrupt its internal state):

https://github.com/cryptosphere/rbnacl/blob/master/lib/rbnacl.rb#L88

Thread.exclusive is useful because it provides an implicit mutex you can ensure is initialized correctly before any other threads start.



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